Phase Instability in Dilute Interstitial Solid Solutions of Nitrogen in TantalumD. P. Seraphim, N. R. Stemple, and D. T. Novick Citation: Journal of Applied Physics 33, 136 (1962); doi: 10.1063/1.1728472 View online: http://dx.doi.org/10.1063/1.1728472 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/33/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhanced interface perpendicular magnetic anisotropy in Ta|CoFeB|MgO using nitrogen doped Taunderlayers Appl. Phys. Lett. 102, 242405 (2013); 10.1063/1.4811269 Change of the superconducting, transport, and microscopic properties of transition metals uponintroduction of interstitial impurities and deformation-induced defects Low Temp. Phys. 27, 345 (2001); 10.1063/1.1374718 Solid-state amorphization at tetragonal-Ta/Cu interfaces Appl. Phys. Lett. 75, 935 (1999); 10.1063/1.124559 Transmission electron microscopy of the sequence of phase formation in the interfacial solid-phasereactions in Ta/Si systems J. Vac. Sci. Technol. A 15, 253 (1997); 10.1116/1.580521 Diffusion of an interstitial solute in a dilute ternary fcc alloy J. Appl. Phys. 45, 4699 (1974); 10.1063/1.1663121

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JOURNAL OF APPLIED PHYSICS VOLUME 33, NUMBER 1 JANUARY, 1962

Phase Instability in Dilute Interstitial Solid Solutions of Nitrogen in Tantalum

D. P. SERAPHIM, N. R. STEMPLE, AND D. T. NOVICK*

International Business Machines Corporation, Thomas J. Watson Research Center, Yorktown Heights, New York (Received June 22, 1961)

An ordered structure is found in interstitial solid solutions as dilute as 0.1 at.% nitrogen in tantalum. On the basis of the symmetry of the ordered structure, tetrahedral rather than the usual octahedral sites are considered appropriate for the nitrogen atoms. The effect of the ordered structure on the properties of the solid solution are discussed.

INTRODUCTION

A N ordered solid solution of 5 at.% nitrogen in tantalum with a lattice parameter of 10.1 A,

slightly larger than three cell lengths of a random inter­stitial solid solution, was reported by SchOenberg.! More recently, Gebhardt et al.2 have shown that solutions richer than 4.5% nitrogen are unstable on quenching from 1500°C, producing residual resistivities which are lower than some of the more dilute alloys. Other evi­dence concerning the superconducting properties may be cited3 which indicates that a reasonable quantity of atomically ordered lattice is present in substantially more dilute tantalum nitrogen solid solutions, perhaps even in solutions as dilute as 0.2 at.%, i.e., in the range of commercial purities. There have also been reports4,6

on anomalous behavior of the Snoek relaxation in the internal friction spectrum of Ta-interstitial solid solu­tions in the range of 1 at.% nitrogen or oxygen. It was inferred from the internal friction data that the inter­stitials were interacting in pairs with neighboring inter­stitials to produce a second relaxation peak distinct from the Snoek effect. From the above observations it is cer­tain that strong interactions are present in these solu­tions which tend to order the interstitials on specific lattice sites.

The purpose of the work reported here was to in­vestigate the presence of the superstructure in single crystals of tantalum. The symmetry and lattice points have been defined by x-ray measurements and some corroboratory experiments have been carried out with electron microscopy. The relation of the various prop­erties of the solid solution to the incidence of the superstructure are discussed.

EXPERIMENTAL

The growth of single crystals of tantalum and the purification of these by heating them in ultra-high vacuum has been discussed previously. 6 Following this

* ~resent address: Department of Metallurgy, Columbia Uni­versity, New York, New York.

1 N. SchOenberg, Acta Chern. Scand. 8, 199 (1954). 2 G. Gebhardt, H. D. Seghezzi, and W. Dtirrschnabcl, Z.

Metallk. 49, 577 (1958). 3 D. P. Seraphim, Solid State Electronics 1, 368 (1960). 4 R. W. Powers and M. V. Doyle, Acta Met. 4, 233 (1956).

preparation, the residual interstitial impurities of the specimens can be estimated from the resistivity at helium temperatures (10-3 J.lOhm-cm) to be much less than 1 ppm. This was the type of specimen which was subsequently loaded with nitrogen in situ in the vacuum system. The nominal content of nitrogen was established by controlling both the pressure of nitrogen in the vacuum and the temperature of the specimen. The lowest equilibration temperature (highest nitrogen con­centration) was 2200oK, which should be sufficient to ensure homogeneous solution in the 20-min interval employed for loading. The data of Andrews7 were useful in the calculation6 of the equilibrium concentrations at fixed P and T, and the temperature independent resis­tivity was checked with the data of Gebhardt et al.2

The resistivity of the specimens at helium tempera­tures, which increased by 5.1 Jl. ohm cm for each atomic percent nitrogen,2 was used to check the nominal con­tents. The discrepancies, as large as 10% of the nominal composition, could be accounted for by a much smaller uncertainty in the temperature of the specimen during the gas absorption.

RESULTS

Superconducting Properties

The superconducting properties of tantalum have been investigated with two techniques; by monitoring the specimen resistances as a function of magnetic field, and by measuring the magnetic induction with a ballistic galvanometer also as a function of magnetic field. The induction method measures directly the volume of superconducting (i.e., diamagnetic) material while the resistance technique is extremely sensitive to short­circuiting superconducting paths which may occupy only a small fraction of the total volume of the speci­mens. While both types of transition have been found sensitive to the addition of nitrogen to tantalum3,8 the resistance transition is profoundly influenced by thermal ~reatment of the specimens (in vacua of 10-8 mm Hg) m the range of temperature where nitrogen is mobile enough to move in the order of 1000 A during the anneal­ing time. Typically, the transition, rather than occurring at a well-defined magnetic field (the critical field),

• R. W. Powers and M. V. Doyle, Trans. Am. Inst. Mining, Met., Petrol. Engrs. 215, 655 (1959). 7 M R A d J A Ch S 5

6 D. P. Seraphim, J. I. Budnick, and W. B. Ittner III Trans 8" n re,:"s, . m. em. oc. 4, 11, 1845 (1932). , , D. P. Seraphim, D. T. Novick, and J. L Budnick Acta Met Am. Inst. Mining, Met., Petrol. Engrs. 218, 527 (1960). . (to be published). ,.

136

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I'II.\SE I:..JST;\BILITY IN SOLUTIONS OF \1 l~ Ta 137

broaden~ into a continuous increase in resistance wil h increasing magnetic field. The specimens appear 10 be in a state partly superconducting and partly normal over a considerable range of magnetic field. This effect, although not entirely created by the ageing treatment, is certainly accentuated by it Thus it may be inferred that the configurational distribution of nitrogen in the tantalum lattice is significant in creating or supple­menting the structure responsible for the nonideal :iuperconducting behavior. The resistance transitions are influenced by extremely small quantities «0.1 at%) of nitrogen while the induction transitions, because they represent the diamagnetic volume, begin to broaden only for rather more concentrated solutions, i.e., 0.2 at.%. To illustrate the general behavior, the beginning and completion of magnetic field penetration (within a few percent) into a tantalum specimen with 0.2 at.% oxygen is shown in Fig. 1 (b) as a function of tempera­ture. The comparable transition field increment for a pure tantalum specimen is not observable on the same scale and this is represented as a single curve in Fig. 1 (a). The critical temperature is also a function of the nitro­gen content as shown in Fig. l(c). Note that the critical temperature decreases rapidly with nitrogen concen­tration until ~0.2 at.% and then begins to decrease much less rapidly. It is just this range of nitrogen con­tent at which the behavior like that in Fig. 1 (b) becomes prominent. It was this information that prompted the authors to at tempt to find two crystal phases in the dilute solid solution with the use of x rays and electron microscopy.

X-Ray Diffraction

Specimens containing from 0.1 to 4 at.% nitrogen do indeed contain a second phase. With a rotating crystal technique two new layer lines appear between each set of layer lines for the original tantalum bcc lattice. In addition, extra reflections appear on the original layers. A typical pattern of reflections obtained when the specimen was oscillated about the (110) direction is shown in Fig. 2. (A few reflections from misoriented grains are visible on this photo.) The lattice parameter of the second phase is 10.1 A independent of the nitrogen concentration up to 4 at.%. The second phase is thus a superlat tice whose unit cell is equivalent to 27 cells of t he random interstitial solid solution expanded in volume by ~5%. The expansion is greater than this if the comparison is made to I he lattice parameter of ~ 1 % random solid solution rather than a 4% solution. This is because the lattice parameter for the random solid solu­tion expands with increasing nitrogen concentration. Apparently the random solid solution, even in the range from 3 to -1 a1.% nitrogen, is essentially strain free when the superstructure is absent. The sharp reflections for a specimen with 3 at.% nitrogen are such an indication, as is shown in Fig. 3(a). Upon ageing this same specimen overnight at 420°C in vacuum (10-8 mm Hg), the super­structure appeared as in Fig. 3(b) and the lines for

150 >

(a)

To 104

% INTERSTITIALS

°4.0 4.1 4.2 '

J.-, tOO ~"'

b) ~2

(0)

5~ 50 ~

To WiTH 0.2 ATOMIC % N

.';;---;!-,-t;--t4.,~-t.4.4 4.5 4.6 4.7 TEMPERATURE: (~i<)

NITROGEN (ATOMIC %)

FIG. 1. The critical magnetic fields and critical temperatures for superconductivity for pure tantalum and tantalum doped with nitrogen. Figure 1 (a) shows only a single critical field curve for pure Ta. Figure 1 (b) shows that Ta doped with nitrogen has a change in induction over a large interval of magnetic field, an interval in which both the superconducting and normal phases are in equilibrium. Figure 1 (c) shows the change in critical tempera­ture with nitrogen concentration. A change of slope occurs at 0.2 at.% nitrogen.

the tantalum lattice smeared considerably. The line broadening may be due to accomodation strain to fit in the larger atomic volume of superlatlice, but morc likely it is due to the small dimensions of a plate-like structure (see Fig. 6).

The intensities of the superlattice lines tend to in­crease with increasing nitrogen content. However, the superlattice is not present in every specimen nor does it appear upon ageing every specimen with appropriate nitrogen content. The reason for this anomalous be­havior will be discussed in the section dealing with electron microscopy.

FIG. 2. Oscillating crystal x-ray photograph of reflections for Ta doped with nitrogen; rotation about \1101.

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13K S ERA P II T 1\1, S T E 1\1 1'L E, AND :\ 0 VIC K

(a)

(b)

FIG. 3. Oscillating crystal x-ray photograph of reflections for Ta doped \,ith 3 at(:~ nitro;:;en; (a) before ageing and (b) after ageing at -1-20°(" in vacuum.

The symmetry of the superiattice was investigated with a Weissenberg-type camera with which it is possible to select t he various layer lines from the zero levels to the upper levels independently. From three Weissenberg photographs all reflections up to and including the second level were indexed on t he accompanying \Yeissen­berg diagram, Fig. -t. Here is evidence that repetition occurs along three orthogonal axes and that the Yliller indexing must be on the basis of a bel' superlattice cell equivalent to 27 tantalum bcc cells. A three-cell ordering of basic bel' lattice has previously been observed in Ti and Zr base alloysHI but of substitutional rather than interstitial type. The new phase in these alloys (w) is discussed as metastable in the transformation sequence f3 to w to ex. The w phase in Ti Cr (0.08) is hexagonalIl (pseudo-cubic) with ci a=O.622 compared with cia = 0.613 for a cubic cell indexed on an hexagonal lattice. \Yith the present data for Ta there is no evidence for the w structure either in the comparative intensities or in the splitting of appropriate reflections of the compound.

The cubic symmetry is confirmed by Laue photo­graphs with the x-ray beam along (111) and (100). Obviously another symmetry than cubic would not be

\I P. D. Frost, \\'. :\1. Parris, L. L. Hirsch, ]. R. Doig, and C. :\1. Schwartz, Trans .. \m. Soc. :\letals 46,1056--1074 (1954).

10 .\. E. Austin and J. R. Doig, j. Metals 9,27-30 (1957). 11 B .. \. Hatt and ] .. \. Roberts, ,\cta :\let. 8, 575-584 (1960).

represented by a threefold axis for the (111) and four­fold axis for the (100) as shown in Fig. S.

Electron Microscopy

Anomalous transmission effects which might be at­tributed to superstructure in specimens of tantalum foil loaded with nitrogen are illustrated in the series (a) to (c) in Fig. 6. These foils were prepared by a puriflcat ion in vacuum (10-' mm Hg) at 2300°C, after which they were loaded with a desired quantity of nitrogen. They were subsequently thinned by electropolishing. Two specimens in the range of 0.1 to 1 al.% nitrogen pro­vided the structure in Fig. 6, but only after being heated for a time in the electron beam. Thus the sequence in Fig. 6 is taken as a function of time, i.e., t he maze pattern begins, Fig. 6(a); broadens in texture, 6(b); and finally transforms catastrophically with complete de­focusing into an entirely new pattern, 6(c). By straining the specimen while in the beam, i.e., by defocusing and by variation of beam intensity, it is possible to develop the structure 6((') in more finely divided form and earlier in the time sequence. It may be noted that the sequence in Fig. 6 i, almost identical to that found by :\uttingI~ in the system Fe-~i-Co, where there is an ordering re­action followed by a martensitic transformation. Ho\\,­ever, the diffraction pattern from the same area, Fig. 7, does not show the superstructure found by x-ray tech­nique in the wire specimens. Instead there is a profusion of satellite reflections. l'nfortunately, we did not get a

OBSERVED REFLECTIONS FOR T027N (APPR()X) ON SUf-ERIMPOSEO WEISSEN8ERG NETS.

[110] AXIS OF HQlATION

o ZERO LAYER (/l~ln

o FIRST LAYER (~.,h+I,2l

h. SEcor~D LAnR (h.h+~. 1)

A FEW REFLECTIOfJS HAVE lJefN IrHA-.X~ U MEFl:ELY iU SI,UW THE PATTER~J OF INDEXlfJG

FIG. 4. \Veissenl>erg diagram for the ordered structure in Ta doped with ~-l at.'/;; nitrogen, indexed in cuilic notation.

12 ]. Nutting (private communication).

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" II ,\ S E J:\ S '1'.\ 11 I I.l T Y 1:\ SOL t' T J 0 ~ S () F ~ T:\ T a

diffraction pat tern early enough in the sequence, i.e., one representing the ordered or maze pattern in 6(b) rather than the needle-like structure. Nonetheless, the structure in Figs. 6(a) and 6(b) clearly appears to be of \YidmansHit ten type and could be t he ordered structure with which we are concerned. The spacing of the dark sections (anomalous transmission) is much less than son A. Consequently, the reHections would be consider­ably broadened, and this effect in conjunction with the small amount of ordered material, may weaken the intensity to extinction. Perhaps this is why we could not always get a diffraction effect with x rays, and in the present case failed, with electrons, to get a distinctive diffraction pattern representing long-range order.

A large number of specimens of pure foil thinned by electropolishing wefe also investigated with electron transmission. ;\;one of these specimens illustrated the structure shown in Fig. 6. Thus it appears that carbon, which may deposit on the specimens in the microscope, did not create the structures.

(a)

(b)

FIG. 5. Laue x-ray photographs of the ordered structure in Ta doped \\'ith~4 at.% nitrogen; (a), x-ray beam along (111), (u 1, x-ray beam along {100).

(a)

(b)

(e)

F£G. 6, Electron transmission microscopy photographs of Ta (hped with ~O,3 at.';{ nitrogen. The sequence is described in the text. Magnification: (a) -96000 times, (h) -64000 times. and (cl -40000 times.

DISCUSSION AND CONCLUSIONS

An interpenetrating bcc superlattice of 27 cells forms in the tantalum lattice with nitrogen concentrations in the range of 0.1 to 4 at.%. It may be inferred that the random solid solution is unstable in this range of com­position and that the incidence of the superstructure is dependent largely upon the thermal history of the speci­men. The fact that x rays are able to distinguish the presence of such a structure is indeed remarkable, since

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t..j.() S ERA P II 1 1\1, S T E:'I P L E, .\:\]) :\ 0 V I C K

(a)

(b)

FlG. 7. Diffraction pattern of an area showing the same mi~ croscopy patterns as Fig. 6; (a) microscopy pattern, and (h) diffraction pattern. Magnification: (a) -42000 times. The diffraction pattern was taken after a structure like that in 6(c) appeared.

the diffracting power of the nitrogen atoms in this con­centration range is completely negligible. Therefore, we may interpret the diffraction effect as due to static dis­placements of the tantalum atoms as affected by the nitrogen atoms near them. The regular periodic dis­placements of the tantalum atoms giving rise to super­lal tice reflections is then evidence for the regular periodic arrangement of nitrogen atoms. If we argue in this way we are able to construct the unit cell for the ordered structure and to write the concentration of the pseudo­compound. Two such structures are shown in Fig. 8; (a) employing octahedral sites for nitrogen atoms, and (b) employing the alternative tetrahedral sites.

In either case the structure is body centered and con­tains 3 tantalum cells along an edge or 27 cells in all. Either structure will diffract with layer line repetition, as shown in Fig. 2. However, (a) is inherently tetragonal due to the tetragonal symmetry of the octahedral sites, i.e., each nitrogen atom has but two near neighbors of tantalum and the three atoms together lie along one of the lOO-type directions. The superlattice in this case would have 3 antiphase possibilities. The fact that the superstructure is cubic rather than tetragonal favors the selection of the (b) type of cell. Note that the nearest neighbors to the nitrogen atom in this cell actually define the corners to a tetrahedron, i.e., a cubic structure with fourfold inversion symmetry. In either case since there

are 2 nitrogen atoms per 27 tantalum cells, each contain­ing 2 tantalum atoms, the pseudo-compound is Ta27i\". There is no v·,-ay to determine if all of the nitrogen sites are filled or to what extent the structure is defective in arrangement of the nitrogen atoms. [It is worthwhile to compare the structure here to the various hydrides of the transition metals as determined by neutron diffrac~ tion. It may be noted from several of the following illustrative references that hydrogen often seeks tetra­hedral sites in an ordered arrangement, and also forms a defect lattice in the sense that all sites are not filled (Libowitz,13 Rundle et al.,14 Sidhu,15 and Roberts16)]'

However, near Ta27:\ the solid solution appears to be completely unstable and gives rise to superstructure lines as intense as those from the original tantalum lattice (see Fig. 2).

The ot her possibility which should perhaps be dis­cussed concerns keeping the cubic symmetry by filling each cell of the ordered structure (27 basic cells) with 6 nitrogen atoms in octahedral sites, i.e., Ta27X:j. Two nitrogen atoms are then in each directional type of site. Since the x-ray intensities peak at around 4% rat her than at 11% nitrogen, this structure is not favored.

Some concluding remarks are appropriate concerning the selection of tetrahedral rather than octahedral sites for interstitials in tantalum. The actual selection of

'" x « o « a: >­w >-

o

o 000 OCTAHEDRAL SITES {WO 1/6) l.

BODY CENTERED SITES {l12, 1/2, 4/6)J

INTERPENETRATION OF NITROGEN

IN Ton N (371 ATOMIC %)

0

0 I \ /

: )( 0 1/ \

~ -\ .;:.. 0 ij \

"" \

0 0 0 0 TETRAHEDRAL SITES{(O.1/12.1/6) }

BODY CENTERED SITES (l/2,7/12,A/6)

INTERPENETRATION OF NITROGEN

IN T027 N (3.71 ATOMIC %)

FIG. 8. Two Jlossible ordered cell structures for TanN; (a) placing nitrogen in octahedral sites, and (b) placing nitrogen in tetrahedral sites.

13 G. G. Libowitz, Nuclear Materials 2, 1-22 (1960). 14 R. E. Rundle, C. G. Shull, and E. O. Wollan, Acta Cryst. 5,

343 (1952). 10 S. S. Sidhu, L. Heston, and D. D. Zauberis, Acta Cryst. 9,

607 (1956). 16 R. \V. Roberts, Phys. Rev. 100, 1257 (1955).

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PHASE INSTABILITY IN SOLUTIONS OF N IN Ta 141

octahedral sites was originally suggested by the tetrago­nality of martensite in the iron system.17 ,18 The ani­sotropy of the Snoek effect in this system was in agree­ment with the x-ray evidence. According to Zener,18 the selection of octahedral sites is dictated by the strain energy, for although the volume of the tetrahedral sites is greater, the anisotropy is such that the deformation in the octahedral sites is along the (100) direction which is soft elastically. In the tantalum system the octahedral holes are '" 14% larger than the same holes in iron. Furthermore, the heat of solution is negative rather than positive so that strain energy is no longer the major factor in determining the equilibrium solubility. Again drawing analogy to hydrogen, there is reason13 to believe in substantial ionic and perhaps even some covalent type bonding. Since we are aware of no apriori reason for selecting octahedral sites for interstitials in tantalum, we should favor the tetrahedral sites on the basis of cubic symmetry of the ordered structure. Unfortunately, even with 4 at.% nitrogen in the tantalum lattice there is insufficient diffracting power to define the nitrogen positions with certainty. A Patterson projection in the (110) plane did not indicate a Ta-N vector in either the octahedral or the tetrahedral site.

'In a sense, the conclusions here are complementary to the results of Powers4 ,5 concerning the pair-wise inter­action of oxygen or nitrogen in the tantalum lattice.

Indeed, the ordering is a direct result of the pair-wise attractive interaction of the interstitial atoms, but once ordered, we would expect the Snoek effect to disappear. However, the solution is likely not completely ordered and the disordered portion can still contribute to in­ternal friction. Powers actually observed a gradual de­crease in both the interaction peak and the Snoek peak on ageing his specimens in the range of 400°C.

We would now emphasize that the brittle nature of interstitial solid solutions in the range of 0.1 at.% and greater may not be due to atmospheres of interstitials around dislocations. Instead we would propose that the ordered structure (when it is present) must present a large barrier to dislocation motion. It seems impossible even to visualize a dislocation with a Burgers vector suitable for simple slip through a structure with such a large unit cell. Thus, to drive a dislocation through the ordered region, the ordering energy must be supplied for each nitrogen atom dislocated from the superstructure relation. Catastrophic failure can be expected before appropriate stress levels are exceeded.

ACKNOWLEDGMENTS

The authors have appreciated helpful discussions with M. Miksic and A. S. Nowick. In particular we thank H. Cole for many helpful comments, and W. Heller for drawing our attention to some of the neutron diffraction studies concerning hydrogen in the transition

17 N. J. Petch, J. Iron and Steel lnst. No.1, 221 (1943). I DC' d . 18 C. Zener, Elasticity and Anelasticity of Metals (University , m~ta s. . ameron asslste WIth the electron

of Chicago Press, Chicago, 1948), pp. 117-119. \t-.. mlCroscope.

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